1. Field of the Invention
The invention relates to short take-off and landing aircraft and vertical takeoff and landing aircraft (STOL/VTOL A/C). More particularly, the invention relates to a system and method for combining and transferring power between an electric fan engine and an internal combustion engine to maximize take-off, horizontal and hovering flight performance.
2. Description of the Related Art
FIGS. 1A-1C illustrate perspective views of several typical aircraft with tilt engine capabilities. The use of tilt engines in aircraft is well known to those of ordinary skill in the art. The principle benefit of a tilt engine is a shortened or vertical take-off capability. Vectoring the thrust allows the aircraft provide a vertical component of thrust instead of, or complementary to, “conventional” horizontal thrust. Horizontal thrust pushes the aircraft forward, thereby moving air over the lifting surfaces of the aircraft, generating lift according to the well known principles of Bernoulli, and flight is thus achieved.
Aircraft with engines that can tilt seek to shorten their take-off distance, or eliminate it completely, by vectoring the thrust from the engine to a substantially total or partially vertical orientation. Vertical thrust, as in a rocket or helicopter, lifts the aircraft straight up, or shortens the take-off distance considerably. Following take-off, the tilt engine aircraft typically rotates the engines to a substantially horizontal position to push the aircraft forward, thereby moving air over the wings, as discussed above. The transition from vertical to horizontal flight must be done carefully, especially in non-rotating wing aircraft, because too fast a transition will leave the aircraft with an insufficient amount of air moving over the wings, and no lift will be generated, and the aircraft will fall, like a rock.
Another problem that all the aircraft shown in FIGS. 1A-1C exhibit is that of high observability. Observability is defined as the ability for the aircraft to be “observed” or detected by detection equipment. The detection equipment in this case refers to radar, which detects objects using the radio portion of the electromagnetic spectrum, and infra-red detectors, which uses the infra-red (IR) portion of the electromagnetic spectrum to detect objects. Generally, the larger an object is, the easier it will be to detect using radar. Detectability by radar can be lessened by designing the object to deflect electromagnetic energy in different directions (i.e., not back towards the radar receiver), use of non-reflective or low-reflective materials, or making the object smaller.
Infra-red detectors detect the object by the heat that the object emits. All objects emit heat (otherwise known as “blackbody radiation”). The heat generated by a body shows up in the infra-red portion of the electromagnetic portion. The main source of heat in an aircraft are the engines, and they are usually good heat generators. Turbo jet engines operate at very high temperatures (from about 1000° F. to as much as 2700° F.). Of course, the high temperature portions of the engines are covered with other components, which helps to hide the heat, and the cold atmosphere assists in reducing the heat of the engine, but, the engines still create enough heat to be highly observable to infra-red detectors.
The aircraft illustrated in FIGS. 1A-1C are highly detectable in both a radar sense and an infra red sense. The opposite of a high detectability is low detectability, and an aircraft that has low detectability can also be said to have low-observability, or “LO”. An aircraft that has LO is also said to be “stealthy”. The aircraft of FIGS. 1A-1C are not stealthy from a radar perspective, because their engines have a large radar cross section (RCS) that adds to the RCS of the main fuselage of the aircraft. Radar cross section is a factor that relates the amount of power of the radio waves that an object reflects or scatters back in the direction of the radar to the power density of the radar's transmitted waves at the object's range. The radar cross section is dependent on the cross sectional area of an object as seen by the radar, the object's reflectivity and its directivity. The cross sectional area, of course, is directly related to the size of the object. The aircraft of FIGS. 1A-1C also suffer from high infra-red signature, because their engines, which are a large source of heat as discussed above, are distanced away from the fuselage and are thus easy to observe from substantially all angles.
All of the examples shown and discussed above in regard to FIGS. 1A-1C use the same engine for both vertical and horizontal thrust. Generally, this is a relatively inefficient design. A substantially larger amount of thrust is needed to vertically launch the aircraft, than is needed, or even desired, to maintain horizontal flight. If loiter operations are considered, the problem is even more pronounced. That is because gas turbine engines, and especially turbo jet and turbo fan engines, operate with greater efficiency at a certain operating condition. If a gas turbine engine has been designed to produce a large amount of thrust for vertical take-off, it will not operate efficiently with lesser amounts of thrust for horizontal flight, and will operate with even less efficiency at loiter speeds.
Therefore, another class of STOL/VTOL A/C has been developed in which either two engines are used to provide vertical and horizontal thrust, or complicated gearing is used to transfer power from one engine to a vertical propulsion system and a horizontal propulsion system. For example, U.S. Pat. No. 5,890,441 to Swinson et al. (the “Swinson” Patent) discloses a semi-autonomously directed, autonomously controlled, gyroscopically stabilized, horizontal or vertical take-off and landing (HOVTOL) flying apparatus that employs two vertical lift devices equally and longitudinally spaced from the center of gravity of the aircraft. Enclosed within the aircraft is a continuously integrated drive train apparatus. There can be one or more means for providing power. Connected to the power means and the vertical lift devices are horizontal thrust devices. Swinson uses the integrated drive train apparatus such that when the power system rotates the drive train, the vertical lift apparatus and horizontal thrust apparatus are caused to counter rotate at right angles, simultaneously providing both vertical lift and gyroscopic roll stability, and simultaneously providing both horizontal thrust and gyroscopic yaw stability during flight. FIG. 1 of Swinson illustrates the complex drive train assembly needed to provide power from the power means to both the vertical and horizontal lifting devices.
U.S. Pat. No. 5,823,468 to Bothe (the “Bothe” Patent) discloses a hybrid aircraft that has a lifting body hull and four wing sections arranged in tandem that are pivotally moveable about their neutral axis. As shown in, and described with respect to FIG. 11 in the Bothe patent, Bothe discloses using a gas turbine engine to create electricity to drive at least four propellers. The propeller engines are tiltable to provide vertical and horizontal thrust. In a second embodiment (FIG. 13), Bothe discloses using a series of fans in a fixed vertical orientation from commercially available jet engines driven by electric motors to provide vertical thrust, and horizontal electric fan engines to provide horizontal thrust. Thus, Bothe discloses electric fan engines to provide both vertical and horizontal thrust.
U.S. Pat. No. 4,828,203 to Clifton et al. (the “Clifton” Patent) discloses a vertical and short take-off and landing aircraft comprising a fuselage, a set canard wings, a set of lift fan wings, air deflectors, lift wings, and a pusher propeller. The lift fan wings comprise a generally circular duct extending vertically through the wing, a multi-bladed fan mounted for free rotation axially in the duct, and an internal combustion engine connected to the fan for selectively applying rotational torque to the fan. The air deflectors are arranged about the lift fan wing in a louver-type of system for directing even flow of air to the fan. The lift wings are attached to the fuselage aft of the center of gravity of the aircraft and generally at a location vertically higher than the lift fan wings. The pusher propeller is connected to the internal combustion engine and attached to the fuselage aft of the lift fan wings.
Finally, U.S. Pat. No. 4,125,232 to MacLean et al. (the “MacLean” Patent) discloses a small jet aircraft that has pitched horizontal rotor blades to provide vertical lift, and a conventional jet engine for horizontal flight, the rotor blades being located within openings formed through the wings. The aircraft of MacLean includes two piston engines 13a for vertical lift and one jet engine 13b for horizontal flight.
Thus, the prior art of aircraft that are characterized as STOL/VTOL A/C suffer from either extremely complicated means of transferring power from one engine to both vertical and horizontal propulsion systems, or use only internal combustion engines to provide both vertical and horizontal engines, thereby being inherently inefficient in their operation.